Modern integrated circuits (ICs) are composed of multiple layers of conductors and substrate materials, such as insulators and semiconductors. Inspecting and editing a circuit or other hidden interior feature in an IC requires navigating to the target area and milling through one or more of the multiple layers of substrate material. Circuit Edit (CE) reduces IC development costs by reducing the number of mask sets that are required during the design-debug phase, and speeds overall time-to-market.
Most CE activities today are performed with Focused Ion Beam (FIB) systems, which are commonly used to mill away a substrate material to expose hidden features and also deposit materials with high precision. These capabilities can be used to cut and connect circuitry within a device, as well as to create probe points for electrical test. Applications include validating design changes, debugging and optimizing devices in production, and prototyping new devices without costly and time-consuming mask set fabrication.
Typically material removal in FIB systems is accomplished by using beams of relatively large ions to physically sputter away the substrate material. Most FIB systems use gallium ions produced by a Liquid Metal Ion Source (LMIS) because such sources are easy to fabricate, operate at room temperature, and are reliable, long lived, and stable. Ion sources using indium are also known.
In LMIS systems, it is also known to use alloy sources comprising metal alloys of two or more different elements. Prior art alloy sources are typically equipped with mass filters so that the desired ion species can be selected. Alloy sources are often used because the desired ion species alone would be unsuitable for use in a LMIS (for example when the elemental species has a too high melting point) but the properties of the alloy are more favorable. Alloy sources have also been used to switch between two desired ion species for implantation, such as using an alloy source producing beryllium and silicon ions to implant p-layer and n-layer structures, respectively, on a gallium arsenide substrate.
Plasma ion sources have also been used to form ion beams. The magnetically enhanced, inductively coupled plasma ion source described in U.S. Pat. App. Pub. No. 2005/0183667 for a “Magnetically enhanced, inductively coupled plasma source for a focused ion beam system” can be used to produce a finely focused beam with a relatively large beam current that can be used for CE applications.
Although FIB systems can also be used to generate a sample image while milling in order to monitor the milling process, the image is typically restricted to the very top surface of the sample. This causes problems for CE applications because many modern ICs do not include visible surface features to serve as reference points for navigation. This is especially true for backside editing, which is becoming increasingly common for CE. Instead of trying to mill through many layers of dense circuitry from the front, operators turn the device over and mill through the substrate silicon to access target areas from the back.
FIG. 1 shows a schematic representation of a typical prior art backside IC device 10. As shown in FIG. 1, a solid layer of silicon 12 typically covers the backside of the circuitry. Underneath the silicon layer, the IC device shown in FIG. 1 includes an active region 14 and a number of deeper metal layers M1 through M5, with each layer including metal lines 16 and vias 18 surrounded by a dielectric material 20. FIG. 2 shows a schematic representation of the backside IC device of FIG. 1 after a wedge polish, which is an angled polish that exposes multiple layers at once. In the schematic view of FIG. 2, it can be seen that the wedge polish has removed all of the silicon and active layers and exposed portions of metal layers M2 and M3. In a top-down view of a sample such as the one shown in FIG. 2, as the sample is viewed from left to right, in area 22 a via and portions of the dielectric from layer M2 would be visible; in area 24 a portion of the M2 metal line would be visible; in area 26 portions of vias surrounded by dielectric would be visible; and finally in area 28 the metal line of layer M3 would be visible.
A wedge polish as shown in FIG. 2, while a convenient way of looking at multiple layers at once, cannot be used for actual CE because the IC device is destroyed. For CE of features hidden beneath a sample surface, such as found in backside edits on bulk silicon samples, it is typically necessary to precisely determine the location of a desired buried feature and then to mill away substrate material in order to expose that feature. Unfortunately, it can be very difficult to locate such hidden features precisely. Even when the beam is positioned correctly, it is often difficult to expose the features without damaging the features with the ion beam. Once the features are visible in the FIB image, some degree of damage has already taken place. In other words, when using FIB imaging to determine when a feature is exposed and milling should be stopped, often referred to as end-pointing, the feature can be damaged or even destroyed before the milling can be stopped. Moreover, in order to find reference points in an image to determine where on the circuit the feature of interest is located, it is sometimes necessary to expose by trial and error a relatively large area, potentially damaging each area that is exposed.
In one method for navigation on a bulk silicon device, after a sample substrate has been sufficiently thinned by ion milling, it is sometimes possible to visually differentiate highly doped wells from the rest of the substrate in a FIB image. The outline of these doped regions can be useful for navigational purposes. But during backside milling on bulk silicon devices, it is easy to miss the signal from the emerging doped-wells, which can lead to over-milling and damage to the sample. The buried oxide surface itself is very thin and fragile, and the signal from the buried features is also weak and fleeting. Therefore an aggressive high beam current and/or a long dwell time is required to distinguish the transistor wells, which can even further damage the sample.
Real-time imaging using a separate electron beam is another method for determining end-pointing. U.S. Pat. No. 7,388,218 to Carleson for a “Subsurface Imaging Using an Electron Beam,” which is assigned to FEI Company of Hillsboro, Oreg., the assignee of the present application, and which is incorporated herein by reference, teaches an electron microscope that can image subsurface features. The electron beam imaging concurrently with the ion beam allows real time viewing of the milling process for end-pointing, and the ability to view subsurface images gives a much greater margin of error when exposing delicate buried features. Unfortunately, the dual-beam system of Carleson suffers from a number of inherent shortcomings. A dual-beam system is necessarily more complex and expensive than a single beam system. Additionally, it is quite difficult to keep both beams focused to the same focal point, which also introduces error into the system. Although systems using coincident and even coaxial ion and electron beams are known, such systems are complex and still include a degree of inaccuracy that it undesirable for many modern CE applications.
The use of helium ions for subsurface imaging is described by Reiche et al. in “Applications of Helium Ion Microscopy in Semiconductor Manufacturing,” MICROSCOPY AND ANALYSIS, pp. 11-14 (July 2009). However, helium ions are not suitable for milling applications because of the small size of the ions (and the corresponding lack of physical sample damage that they cause). The helium ion beam of Reiche would have to be combined with a separate ion beam column using larger ions for any significant material removal, and thus would suffer from the same disadvantages as discussed above with respect to Carleson.
Thus, there is still a need for an improved method for imaging and processing samples using FIB systems that allows both for rapid, high accuracy navigation and end-pointing and for rapid material removal once a feature has been located.